Yeah and......????Wikipedia tells me that Nog got his commission as an ensign between Behind the Lines and Favour the Bold, effectively the 5th episode of season 6. Valliant was episode 21 of season 6.
That little ferengi was not only the highest ranking officer aboard who's rank had been ratified by Starfleet Command, he was the only officer aboard that ship who's rank had been ratified by Starfleet command.
Nog wasn't a leader, he was a follower.
That's what his whole speech about being part of a group and respecting command to Jake was all about. Even when the ship was expoding all around them and the First Officer and trouble taking control, did Nog step up? No, he didn't because regardless of rank it's not in his personality to lead.